Hexavalent chromium, Cr(VI), is widely used in numerous industrial processes, including chrome pigment production, chrome plating, stainless steel manufacturing, and leather tanning, etc. Epidemiological studies have reported a high incidence of lung cancer among chromium workers exposed occupationally to Cr(VI) by inhalation. Although multiple mechanisms were suggested to contribute to Cr(VI) carcinogenicity, the molecules and pathways that mediate Cr(VI) induced lung cancer are not well understood. Hedgehog (Hh) signaling is a key pathway that plays important roles in the formation of multiple tissues during embryogenesis and in the maintenance of stem cell populations in adults. Dysregulation of the Hh signaling pathway was found in a variety of human cancers, including basal cell carcinomas, medulloblastomas, colon cancers, pancreatic cancers, and lung cancers. Our preliminary data demonstrated a significant decrease in mRNA levels of hedgehog-interacting protein (HHIP), a downstream target and a natural antagonist of Hh signaling, in Cr(VI) transformed cells. Interestingly, Hh signaling was altered in these cells a evidenced by increased levels of downstream target genes, such as Gli1 and Ptch1, suggesting a potential role of Hh signaling in Cr(VI) induced cell transformation and carcinogenesis. The primary focus of this research proposal is to determine the role of Hh signaling in Cr(VI) induced cell transformation and to dissect the mechanism by which Cr(VI) modulates Hh signaling. First, to determine whether decreased HHIP levels and enhanced Hh signaling contribute to Cr(VI) induced cell transformation and carcinogenesis, we will modulate the levels of major components of Hh signaling pathway and analyze the changes in cell proliferation, anchorage-independent growth and in vivo tumor formation. Next, to dissect the epigenetic mechanism underlying the down-regulation of HHIP in Cr(VI) transformed cells, we will analyze the histone modifications and DNA methylation in HHIP promoter. Our proposed study may be the first to link chronic Cr(VI) exposure to the dysregulation of the Hh signaling pathway. The success of this research proposal will significantly improve our understanding of the changes in Hh signaling cascade after Cr(VI) exposure and may provide new insights for developing a better therapeutic strategy for Cr(VI)-related human cancer.

Public Health Relevance

Hexavalent chromium [Cr(VI)] is a potent human carcinogen. Occupational exposure has been associated with increased risk of respiratory cancer. Our proposed study investigates the role of Hh signaling in Cr(VI) induced cell transformation and carcinogenesis. The success of this research proposal will significantly improve our understanding of the molecular basis of Cr(VI) induced carcinogenicity and will provide new insights for developing a better therapeutic strategy for Cr(VI)-related human cancer.